A Holiday update has been released for Outlook 2007 which contains holidays for 2013 and later.
Currently the update is only available as a Hotfix but you can expect it to be included in a regular update later as well.
After requesting and installing the hotfix, you must reimport the holidays to actually get the newly added holidays in your Calendar:
Tools-> Options-> button Calendar Options…-> button Add Holidays
To prevent duplicates you can remove your existing holidays first by changing your Calendar view arrangement to “By Category” (all holidays are in the “Holiday” category):
View-> Current View-> By Category